This invention relates to cooling systems and, more particularly, to cooling systems for use in gas turbine engines.
Cooling of high temperature components in gas turbine engines is one of the most challenging problems facing engine designers today, particularly as it relates to the turbine portions of the engine where temperatures are most severe. While improved high temperature materials have been developed which partially alleviate the problem, it is clear that complete reliance on advanced technology materials will not be practical for the foreseeable future. One reason is that these advanced materials contemplate expensive manufacturing techniques or comprise alloys of expensive materials. Thus, the product, though technically feasible, may not be cost-effective. Additionally, as gas turbine temperatures are increased to higher and higher levels, it is clear that no contemplated material, however exotic, can withstand such an environment without the added benefit of fluid cooling. Fluid cooling, therefore, can permit the incorporation of more cost-effective materials into present-day gas turbine engines and will permit the attainment of much higher temperatures (and, therefore, more efficient engines) in the future.
Various fluid cooling techniques have been proposed in the past, commonly classified as either convection, impingement or film cooling. All of these methods have been tried in gas turbine engines, both individually and in combination, utilizing the relatively cool pressurized air from the compressor portion of the engine as the cooling fluid. Such prior art concepts are discussed in U.S. Pat. 3,800,864-Hauser et al, which is assigned to the assignee of the present invention. One problem associated with fluid cooling is to reduce the system losses, thereby reducing the quantity of propulsive fluid (air) utilized for such nonpropulsive purposes. In the current practices, where fluid cooling has been utilized to augment the inherent high-temperature material characteristics, it has been necessary to absorb the performance penalty incurred when the coolant is injected back into the propulsive stream at locations where performance losses result. For example, it is not uncommon to find that in fluid-cooled turbines the coolant is discharged at some high Mach number region downstream of the nozzle throat such as the nozzle band trailing edge. This type of mixing of the low velocity coolant with the high velocity hot gas stream leads to momentum losses which produce performance penalties.